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Athens Diagnostic Laboratory and Department of Veterinary Pathology, College of Veterinary Medicine, University of Georgia, Athens, GA
| Abstract |
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Key words: Adjuvant; ferrets; fibrosarcoma; oncogenesis; rabies; ultrastructure; vaccination.
In cats, a relationship between vaccination and sarcoma development was first suggested in 1991.6 An increased tumor incidence was observed during the following 5 years, and the feline vaccine-associated sarcoma subsequently became a well-defined clinical entity.12 Factors suggested to have contributed to the increased sarcoma development in the early 1990s include increased use of killed-virus adjuvanted rabies vaccines, the development and use of feline leukemia virus vaccines, and increased subcutaneous vaccine administration.1,12,14 The precise mechanisms by which vaccination promotes sarcoma development in cats is unknown, but it is considered likely that chronic inflammation is important.2,10
Histologic,1,2 ultrastructural,11 and immunohistochemical5 features of feline vaccine-associated sarcomas have been reported. When compared with nonvaccine-associated sarcomas, vaccine-associated sarcomas are significantly more frequent in the subcutis.2 They also are more likely to contain areas of necrosis,2 peripheral lymphocyte aggregates,2,11 cellular pleomorphism,2 and blue-gray material within macrophages.11 Using electron microscopy, this intrahistiocytic crystalline material was identified as aluminum based.11 A partial myofibroblast differentiation of neoplastic cells has been demonstrated in some vaccine-associated sarcomas using immunohistochemistry and electron microscopy.5,11
Cats, dogs, and ferrets routinely receive killed-virus rabies vaccinations. However, to the authors' knowledge, vaccine-associated oncogenesis has only been reported in cats. There are an estimated six million ferrets in the United States, making them the third-most numerous interactive pet.16 Ferrets are recommended to be vaccinated against rabies at 3 months of age and then annually.17 Ferrets also are recommended to receive modified live, nonferret cell cultureorigin canine distemper vaccine.17 This vaccine is initially given at 9 and 11 weeks of age and is then repeated every 23 years.17 Ferrets are routinely vaccinated in the subcutaneous tissues of the interscapular region (S. Hernandez-Divers, personal communication). Although a ferret vaccination-site fibrosarcoma (VSF) was reported in a letter to the Journal of the American Veterinary Medical Association,15 no histologic description of the tumor or evidence supporting an association between vaccination and tumorigenesis was included. The aim of the present study was to determine whether an increased proportion of ferret subcutaneous sarcomas develop at sites used for vaccination. The histology, immunohistochemistry, and ultrastructure of the ferret VSFs were then compared with feline vaccine-associated sarcomas.
| Materials and Methods |
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Histologic examination of the neoplasms was performed using 4- to 6-µm-thick hematoxylin and eosinstained sections of formalin-fixed, paraffin-embedded tissue. Immunohistochemistry was performed using formalin-fixed, paraffin-embedded 4- to 6-µm-thick sections. Sections were heated for 30 minutes in a 60 C oven, deparaffinized, and stained using a TechMate 500 Autostainer (BioTek Solutions Inc., Santa Barbara, CA). Antibodies against vimentin (Biogenex, San Ramon, CA), smooth muscle actin (Dako Corporation, Carpinteria, CA), and desmin (Biogenex) were used. The avidinbiotinperoxidase complex system (Vector Laboratories, Burlingame, CA) was used to observe all immunohistochemical reactions.
Neoplasms from two cases (ferret Nos. 2 and 3) were examined ultrastructurally. Representative specimens of paraffin-embedded tissue were deparaffinized in xylene, rehydrated, and osmicated for 1 hour. Sections of these were stained with toluidine blue and examined using light microscopy to identify areas for ultrastructural examination. Areas of tumor containing marked cellular pleomorphism, immunohistochemically identified areas of smooth muscle actin expression, and intracellular granular material were selected for examination. Ultrathin sections were then cut using an ultramicrotome with glass knives and stained with uranyl acetate and lead citrate.
| Results |
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Histologically, all VSFs were moderately well demarcated and nodular (Fig. 1). All seven were situated in the subcutis and ranged in size from 10 to 25 mm in diameter. Skeletal muscle fibers of the panniculus muscle were seen ventral to all seven tumors. Three VSFs were not in contact with the panniculus muscle, two abutted the muscle causing localized stretching and distortion, and two infiltrated the panniculus muscle (Fig. 2). One NVSF was situated within the superficial dermis. This tumor was poorly demarcated and sessile. Although the other two NVSFs were within the subcutis, panniculus muscle was not observed in sections of either NVSF. All ferret fibrosarcomas consisted of neoplastic cells arranged in elongated interwoven bundles supported by a moderate fibrovascular stroma. Compared with NVSFs, cells in VSFs appeared less organized, with most tumors containing areas of shorter, more tightly packed bundles along with areas that contained loose whorls. Neoplastic cells in five VSFs showed marked cellular pleomorphism. Larger cells (up to 30 µm) in these neoplasms often contained large, round, central nuclei with prominent, often multiple, nucleoli and large quantities of eosinophilic cytoplasm, whereas smaller cells (typically around 10 µm) more often contained homogenous, elongated, central nuclei and small quantities of faintly eosinophilic cytoplasm. Large oval to round histiocytic cells containing intracytoplasmic basophilic granular material were observed in two VSFs (ferret Nos. 2 and 3) (Fig. 3) and zero of three NVSFs. The cells were interpreted as macrophages and were more frequent toward the periphery of the tumor. All 10 fibrosarcomas had at least one mitotic figure/400x field with up to four mitotic figures/400x field present in some tumors. Multinucleated cells were present in three of seven VSFs but in none of the NVSFs. The multinucleated cells were generally scattered throughout the neoplasm and ranged in number from 1 to 10 per 10 400x fields. Necrosis was observed in all VSFs and two of three NVSFs. Small foci of necrosis were present throughout five VSFs and one NVSF. However, two VSFs and one NVSF contained more marked necrosis with the formation of a central cavity that was lined by necrotic cell debris. Mild inflammation was seen, associated with necrotic cells in all tumors that contained necrosis. However, five VSFs additionally contained small to moderate numbers of lymphocytes and plasma cells that were not associated with areas of necrosis. These cells were diffusely scattered throughout the neoplasms and also were present as peripheral nodular lymphocytic aggregates (Fig. 4). The peripheral lymphocytic aggregates were numerous and large (30 cells in diameter) in two tumors but less frequent and small (10 cells in diameter) in the other three.
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Ultrastructurally, both examined VSFs were composed of a pleomorphic population of spindle cells that have oval to convoluted nuclei, euchromatin, and prominent nucleoli. Bi- and trinucleate cells with similar nuclei also were observed in both tumors. In both VSFs, occasional spindle cells contained bundles of subplasmalemmal, elongate, thin filaments that were consistent with actin. Rare intracytoplasmic aggregates of dense, needlelike, crystalline material were observed in the VSF from ferret No. 3. Because of the artifacts formed during formalin fixation, the type of cell that contained this material could not be definitively determined.
Ferrets that had VSFs ranged in age from 1 to 9 years with a median age of 4 years. One ferret that had an NVSF was 1 year old. The ages of the other two ferrets with NVSFs had not been recorded.
Follow-up information was available for three ferrets with VSFs and one with an NVSF. One ferret that had a VSF was euthanatized because of tumor recurrence 3 months after excision (ferret No. 4). In two ferrets, no VSF recurrence had been observed at 8 (ferret No. 1) and 12 (ferret No. 6) months after surgical excision. Histologically, all three of these VSFs had appeared completely excised. One ferret that developed an NVSF (ferret No. 10) was euthanatized 6 months after incomplete surgical excision because of tumor recurrence.
| Discussion |
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Histologic features of subcutaneous sarcomas that are suggestive of an association with vaccination have been described in cats.1,2 These features were compared with ferret VSFs and NVSFs. Sixty-nine percent of feline vaccine-associated sarcomas contain lymphocytic inflammation, predominantly in peripheral areas.2 Peripheral lymphocytic inflammation was observed in five of seven ferret VSFs. Intracellular basophilic material, thought to be adjuvant, was present in 22% of feline vaccine-associated sarcomas.1 A similar intracellular basophilic granular material was present in two of seven ferret VSFs. Neither peripheral lymphoid aggregate nor intracellular basophilic granular material was observed in NVSFs. Subjectively, cells in VSFs appeared larger, rounder, and more pleomorphic than cells in NVSFs. Greater cellular pleomorphism has been described as a feature of feline vaccine-associated sarcomas.2 Although the presence of lymphocytic inflammation and basophilic material provides evidence for vaccine association in ferrets, comparison of other features of feline vaccine-associated sarcomas did not provide additional supportive evidence. In cats, a subcutaneous location has been reported to be more common in vaccine-associated sarcomas than in nonvaccine-associated sarcomas.2 In ferrets, all seven VSFs were within the subcutis, however, two thirds of NVSFs also had a subcutaneous location. Cavitating tumor necrosis has been described as a feature of feline vaccine-associated sarcomas.1 Necrosis with the formation of a central cavity was observed in two of seven VSFs but also was present in one of three NVSF. Three VSFs but no NVSFs contained multinucleated cells. Although 50% of feline vaccine-associated sarcomas have been reported to contain multinucleate cells,1 multinucleate cells were not reported to be significantly more frequent in feline vaccine-associated sarcomas when compared with nonvaccine-associated sarcomas.2
Similarities between the immunohistochemical features of feline vaccine-associated sarcomas and ferret VSFs were observed. As has been reported in 64% of feline vaccine-associated sarcomas,1 three of seven ferret VSFs contained neoplastic cells that stained positively with antibodies against smooth muscle actin. Desmin expression has been reported to be less common in feline vaccine-associated sarcomas5 and was observed in only one of seven ferret VSFs. The peripheral distribution of muscle intermediate filaments that was observed in ferret No. 6 has previously been reported in feline vaccine-associated sarcomas.1 The expression of smooth muscle actin in some tumors is suggestive of, as in feline vaccine-associated sarcomas,11 partial myofibroblast differentiation.
The ultrastructural features of the ferret VSFs provide supportive evidence of a role of vaccination in tumorigenesis. The cytoplasmic crystalline material observed in one tumor was morphologically similar to the cytoplasmic aluminum inclusions present in feline vaccine-associated sarcomas.11 These aluminum inclusions are hypothesized to originate from vaccine adjuvant.7 Although the chemical composition of the material was not identified, its presence suggests that a foreign material may have contributed to the development of this tumor. Actin filaments were reported in 16 of 20 feline vaccine-associated sarcomas.11 Filaments that were interpreted as actin were observed in both ferret VSFs.
In cats, additional evidence supporting a causal relationship between vaccination and sarcoma development was derived from epidemiologic studies.8 In the present study, five of the animals that developed VSFs had received a rabies vaccine within the previous 12 months. However, vaccination records were not available for the remaining two ferrets that developed VSFs. Three ferrets also had received vaccinations against canine distemper within a year of developing a VSF. Nonadjuvanted live virus vaccines have not been shown to cause sarcoma development in cats12 and, if ferret fibrosarcomas are associated with vaccination, the rabies vaccine is considered more likely to be implicated than the distemper vaccine.
Cats with vaccine-associated sarcomas have been reported to develop tumors at a younger age than do cats with nonvaccine-associated sarcomas.2 Because the age of only one of the three ferrets that developed NVSFs was known, it is not possible to compare the ages of ferrets with VSFs and NVSFs in the present study.
The involvement, if any, of retroviruses in the development of feline vaccine-associated sarcomas is not yet clear.3,9 To the authors' knowledge, no retrovirus has been identified in ferrets. It is possible to interpret this as evidence against retroviral involvement in the development of ferret VSFs. However, a retroviral infection was strongly suspected to have caused an outbreak of lymphoma in a ferret colony.4 Therefore, it is difficult to definitively exclude retroviral infection as a contributing factor in the development of these tumors in ferrets.
In conclusion, the high proportion of ferret subcutaneous fibrosarcomas that developed in vaccination sites suggests that vaccination may predispose ferrets to local tumor development. Additionally, feline vaccine-associated sarcomas and ferret VSFs share some histologic, immunohistochemical, and ultrastructural characteristics. To the authors' knowledge, this is the first evidence that vaccination may cause neoplasia in a nonfeline species.
| Acknowledgments |
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